Abstract: Taking the coal mining under Modongwang reservoir of Peigou Mine as an example, the safety of mining the first mining face of 31071 and the influence of thickness-limited mining within protective pillar range on the dam are comprehensively analyzed. It is obtained that the maximum height of the water flowing fractured zone is safe for mining. The thickness-limited mining within the protective pillar makes the influence degree of mining on reservoir and dam reduced significantly; the damage level is less than Ⅱ grade. In order to protect the dam, the advancing speed of fully-mechanized working face is analyzed, which is 2.0~3.0 m/d.